applied sciences

Article

Regeneration of Activated Carbons Spent by WasteWater Treatment Using KOH Chemical Activation

Jung Eun Park, Gi Bbum Lee, Bum Ui Hong and Sang Youp Hwang *

Plant Engineering Division, Institute for Advanced Engineering, Yongin-si 17180, Korea;jepark0123@gmail.com (J.E.P.); mnbbv21c@gmail.com (G.B.L.); buhong@iae.re.kr (B.U.H.)* Correspondence: syhwang80@gmail.com; Tel.: +82-31-330-7271

Received: 29 October 2019; Accepted: 22 November 2019; Published: 27 November 2019 �����������������

Abstract: In this study, spent activated carbons (ACs) were collected from a waste water treatmentplant (WWTP) in Incheon, South Korea, and regenerated by heat treatment and KOH chemicalactivation. The specific surface area of spent AC was 680 m2/g, and increased up to 710 m2/g throughheat treatment. When the spent AC was activated by the chemical agent potassium hydroxide (KOH),the surface area increased to 1380 m2/g. The chemically activated ACs were also washed with aceticacid (CH3COOH) to compare the effect of ash removal during KOH activation. The low temperatureN2 adsorption was utilized to measure the specific surface areas and pore size distributions ofregenerated ACs by heat treatment and chemical activation. The functional groups and adsorbedmaterials on ACs were also analyzed by X-ray photoelectron spectroscopy and X-ray fluorescence.The generated ash was confirmed by proximate analysis and elementary analysis. The regeneratedACs were tested for toluene adsorption, and their capacities were compared with commercial ACs.The toluene adsorption capacity of regenerated ACs was higher than commercial ACs. Therefore, it isa research to create high value-added products using the waste.

Keywords: activated carbon; KOH; regeneration; heat treatment; chemical activation; tolueneadsorption; ash removal

1. Introduction

Activated carbons (ACs) have been widely used for the water treatment process, which includesthe removal of odors, heavy metals, and organic compounds. In particular, drinking water treatmentplants (DWTP) and waste water treatment plants (WWTP) require various types of ACs in largequantities [1–3]. Granular activated carbon (GAC) is commonly used in the filtration process ofDWTP [2,4]. This process can effectively remove organic matter, taste and odor-causing compounds.Powdered activated carbon (PAC) is usually used in final stage of WWTP. The treated water shouldmeet the stringent effluent standard before discharging the water to the river. The PAC filtrationprocess efficiently removes pollutants, so should be required in the WWTP [1,2].

Water treatment plants require a large amount of fresh ACs, but this means that large amountsof spent ACs could be generated after losing the adsorption capacity. The spent ACs are classifiedinto hazardous solid wastes, and are thus prohibited to dispose directly into landfill sites [5,6].The regeneration of ACs is more economically and environmentally effective than replacing freshACs [7,8]. Various regeneration methods have been investigated, including wet oxidation, classicalsolvent regeneration, and thermal regeneration [9]. Thermal regeneration in particular has beenwidely used due to its simplicity, low cost, and versatility [7,10,11].

During thermal regeneration, spent ACs are exposed to high temperatures (~850 ◦C) in the inert gas,and followed by oxidizing conditions [12]. When thermally heated, any volatile compounds adsorbedin the pores are eliminated, and less-volatile compounds are decomposed [3,5]. Thus, the specific

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surface area and pores of spent ACs are efficiently regenerated [13]. The surface area of spent ACs usingheat treatments was observed to increase compared with their initial surface area of untreated spentACs [3,14]. This is because metals accumulating in pores and surface during the water purificationprocess could react as catalysts at a high temperature. The accumulation of metals catalyzes thereaction between surface carbon and oxidizing agents, leading to a pore development and increasedsurface area.

Thermal regeneration of spent ACs usually results in a lower carbon yield and ash generation [15].This in turn leads to a negative effect of AC regeneration. The oxidative reaction at high temperaturesresults in the pyrolysis of the surface carbons, which are then gasified and evaporate, leading to thelow carbon yield [3,15]. Ashes are generated during the thermal treatment which usually block thepores and covers the carbon surface [16]. While the carbon atoms on the exposed surface can be usedthe active site for adsorption, the ashes covering the surface will reduce the active sites, as well as theadsorption capacity [17].

In this study, a chemical activation method was used to overcome the limitations of thermalregeneration. The spent ACs from WWTP were regenerated by KOH chemical activation, and comparedto regenerated ACs using the thermal method. Increased surface area of spent ACs using thermalregeneration was highly dependent on the adsorbed materials on the carbon surface and reactionconditions. However, during chemical activation, the chemical agent reacted with carbons on the surfaceand pores, and was not significantly affected by the adsorption materials. Similarly, previous researchhas investigated the chemical regeneration of spent ACs using the reaction between chemical andadsorbed materials [18], but the chemical activation carried out in this study used the reaction betweenchemical agent and surface carbons at the high temperature. In our knowledge, the regenerationof spent ACs using KOH chemical activation was not published previously. The KOH activationof activated carbon was common and widely investigated, but the regeneration of ACs using KOHactivation is a new concept. In addition, the generated ashes on regenerated ACs were removed bythe acid treatment, and then the surface area and pore size distributions were compared with theregeneration ACs without the treatment. The final products generated in this study exhibit enoughsurface area to use the volatile organic compounds (VOC) adsorption facility for purifying the air,which usually requires relatively higher surface area (

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Co., 99.5%), and then washed with 500 mL of water three times. It was called by C-AC-A (Chemicalactivated-AC-Acid wash) in this study.

2.2. Characterization of Prepared ACs

The specific surface areas and pore size distributions of S-AC, H-AC, C-AC-W, and C-AC-Awere analyzed using the Brunauer-Emmett-Teller (BET) and NLDFT/GCMC Method based on theN2 adsorption obtained at 77 K using an BELSORP-max (BEL Japan, Inc.). The content of oxygenfunctional groups on the surface of each sample was confirmed by X-ray photoelectron spectroscopy(XPS, PHI 5000 VersaProbe, Japan). Elemental analysis (EA) was performed for 12 min at 900 ◦Cusing an elemental analyzer (FlashEA 1112 and 2000, Thermo Scientific, Milan, Italy) to determine theelemental contents of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S). The materialsadsorbed on the ACs were analyzed by X-ray fluorescence (XRF-1800, Shimadzu, Japan). For theproximate analysis, dried samples were placed in the furnace (Furnace, Daeheung science, DF-4S) andheated at 950 ◦C for 7 min, and then lowered to 750 ◦C for 10 h. The analysis was using the ASTMstandard method [21]. The results, including the ash, volatile, and fixed-carbon contents within theACs, were considered as percentages of the weight [22].

2.3. Test for Adsorption of Toluene on the ACs

The adsorption of toluene on ACs was studied in the tubular quartz reactor with an inner diameterof 3 cm. 200 mg of ACs (WACs) was put into the reactor and placed between the ceramic fibers(Cerakwool, ©KCC Co., Korea). As shown in Figure 1, the toluene was evaporated through N2bubbling and the again diluted by N2 for controlling its concentration. The concentration of diluenttoluene was 400 ppm approximately, and the total flow was 1 L/min. The concentration of toluenewas analyzed using THC analyzer with FID detector (Polaris FID, PF-200, Italy). Since the outletconcentration of toluene was the same as the initial concentration, toluene was fully saturated on ACs.

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2.2. Characterization of Prepared ACs The specific surface areas and pore size distributions of S-AC, H-AC, C-AC-W, and C-AC-A

were analyzed using the Brunauer-Emmett-Teller (BET) and NLDFT/GCMC Method based on the N2 adsorption obtained at 77 K using an BELSORP-max (BEL Japan, Inc.). The content of oxygen functional groups on the surface of each sample was confirmed by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, Japan). Elemental analysis (EA) was performed for 12 min at 900 °C using an elemental analyzer (FlashEA 1112 and 2000, Thermo Scientific, Milan, Italy) to determine the elemental contents of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S). The materials adsorbed on the ACs were analyzed by X-ray fluorescence (XRF-1800, Shimadzu, Japan). For the proximate analysis, dried samples were placed in the furnace (Furnace, Daeheung science, DF-4S) and heated at 950 °C for 7 min, and then lowered to 750 °C for 10 h. The analysis was using the ASTM standard method [21]. The results, including the ash, volatile, and fixed-carbon contents within the ACs, were considered as percentages of the weight [22].

2.3. Test for Adsorption of Toluene on the ACs

The adsorption of toluene on ACs was studied in the tubular quartz reactor with an inner diameter of 3 cm. 200 mg of ACs (WACs) was put into the reactor and placed between the ceramic fibers (Cerakwool, ©KCC Co., Korea). As shown in Figure 1, the toluene was evaporated through N2 bubbling and the again diluted by N2 for controlling its concentration. The concentration of diluent toluene was 400 ppm approximately, and the total flow was 1 L/min. The concentration of toluene was analyzed using THC analyzer with FID detector (Polaris FID, PF-200, Italy). Since the outlet concentration of toluene was the same as the initial concentration, toluene was fully saturated on ACs.

Figure 1. Schematic of experimental setup used for performing adsorption test.

3. Results and Discussion

N2 adsorption and desorption isotherms of all the curves for S-AC, H-AC, C-AC-W, and C-AC-A at 77 K followed the type IV isotherms (Figure 2a) [23]. In addition, the empty symbols, which depict desorption curves (Figure 2a), had a slightly different pattern with adsorption. As shown in Figure 2b, the pore size distribution of ACs micropore clearly mixed the micropore and mesopores [24].

The quantity absorbed in the y-axis was dependent on the specific surface area. The N2 adsorption quantity of H-AC was slightly higher than that of S-AC, which means the surface area rarely increased, even when thermally treated. The volatile compounds adsorbed on the carbon surface could be removed by the thermal treatment, but the surface area slightly increased. This is because the volatiles within the pores evaporated, while the pores simultaneously shrunk (Figure 2b). With the exception of H-AC, the pores of the ACs existed between 1–3 nm. The pores of H-ACs were distributed between 0.4–0.5 nm and 1–2 nm. This suggests that the pores of H-ACs shrunk, leading to a lower surface area.

Both chemically activated S-ACs had an absorbed quantity higher than that of S-AC and H-AC, indicating that the both C-ACs have larger surface area compared to others. The surface area could

Figure 1. Schematic of experimental setup used for performing adsorption test.

3. Results and Discussion

N2 adsorption and desorption isotherms of all the curves for S-AC, H-AC, C-AC-W, and C-AC-Aat 77 K followed the type IV isotherms (Figure 2a) [23]. In addition, the empty symbols, which depictdesorption curves (Figure 2a), had a slightly different pattern with adsorption. As shown in Figure 2b,the pore size distribution of ACs micropore clearly mixed the micropore and mesopores [24].

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be increased by the broken particles through the chemical activation, leading to the large surface areas. However, the increase of surface area in this study is mainly caused by the increase of total pore volume as shown in Table 1. When the N2 adsorption of C-AC-W and -A was compared, the C-AC-A adsorption was higher than that of C-AC-W. This indicates that the acid treatment of chemically activated ACs effectively removed the generated ashes and the remained chemical agents used in the chemical activation method. The acid treatment removed the ashes and clear the pores, leading to the increase of surface area and pore volume increase simultaneously (Table 1).

Figure 2. Nitrogen adsorption and desorption isotherms (a) and pore size distribution (b) of S-AC, H-AC, C-AC-W, and C-AC-A. Solid and empty symbols represent nitrogen adsorption and desorption, respectively.

Table 1. Textural properties of the S-AC, H-AC, C-AC-W, and C-AC-A.

S-AC H-AC C-AC-W C-AC-A Specific surface area (m2/g) 681 ± 20 709 ± 18 1383 ± 16 1612 ± 9 Total pore volume (cm3/g) 0.38 ± 0.06 0.40 ± 0.06 0.78 ± 0.04 0.85 ± 0.03

The C 1 s XPS spectrum, along with the de-convoluted peaks of prepared ACs included C-H, C-C, C-O, C=O, and O-C=O, and π-π bonding at 283.6–284.2, 284.6–284.8, 285.5–285.6, 286.7–286.8, 288.8–289.0, and 290.6–291.1 eV, respectively (Figure 3) [25,26]. When compared with S- and H-AC (Figure 3 a,b), the peaks of H-AC rarely changed, even when they were thermally treated. However, the peaks of chemically activated ACs were significantly different (Figure 3c,d). The concentration of the C-H peak in C-AC-W increased due to the non-graphitic carbonaceous species generated during the chemical activation. Consequently, the carbonaceous species were removed by the acid treatment, resulting in the disappearance of the peaks (Figure 3d) [27]. C-O and COOH peaks in XPS curves were observed to decrease and increase, respectively, after KOH chemical activation of ACs [28]. In this study, the peaks related to the C-O and O-C=O also decreased and increased, respectively, after the chemical activation (Figure 3c,d; Table 2).

Figure 2. Nitrogen adsorption and desorption isotherms (a) and pore size distribution (b) ofS-AC, H-AC, C-AC-W, and C-AC-A. Solid and empty symbols represent nitrogen adsorption anddesorption, respectively.

The quantity absorbed in the y-axis was dependent on the specific surface area. The N2 adsorptionquantity of H-AC was slightly higher than that of S-AC, which means the surface area rarely increased,even when thermally treated. The volatile compounds adsorbed on the carbon surface could beremoved by the thermal treatment, but the surface area slightly increased. This is because the volatileswithin the pores evaporated, while the pores simultaneously shrunk (Figure 2b). With the exception ofH-AC, the pores of the ACs existed between 1–3 nm. The pores of H-ACs were distributed between0.4–0.5 nm and 1–2 nm. This suggests that the pores of H-ACs shrunk, leading to a lower surface area.

Both chemically activated S-ACs had an absorbed quantity higher than that of S-AC and H-AC,indicating that the both C-ACs have larger surface area compared to others. The surface area could beincreased by the broken particles through the chemical activation, leading to the large surface areas.However, the increase of surface area in this study is mainly caused by the increase of total porevolume as shown in Table 1. When the N2 adsorption of C-AC-W and -A was compared, the C-AC-Aadsorption was higher than that of C-AC-W. This indicates that the acid treatment of chemicallyactivated ACs effectively removed the generated ashes and the remained chemical agents used in thechemical activation method. The acid treatment removed the ashes and clear the pores, leading to theincrease of surface area and pore volume increase simultaneously (Table 1).

Table 1. Textural properties of the S-AC, H-AC, C-AC-W, and C-AC-A.

S-AC H-AC C-AC-W C-AC-A

Specific surface area (m2/g) 681 ± 20 709 ± 18 1383 ± 16 1612 ± 9Total pore volume (cm3/g) 0.38 ± 0.06 0.40 ± 0.06 0.78 ± 0.04 0.85 ± 0.03

The C 1 s XPS spectrum, along with the de-convoluted peaks of prepared ACs included C-H,C-C, C-O, C=O, and O-C=O, and π-π bonding at 283.6–284.2, 284.6–284.8, 285.5–285.6, 286.7–286.8,288.8–289.0, and 290.6–291.1 eV, respectively (Figure 3) [25,26]. When compared with S- and H-AC(Figure 3 a,b), the peaks of H-AC rarely changed, even when they were thermally treated. However,the peaks of chemically activated ACs were significantly different (Figure 3c,d). The concentration ofthe C-H peak in C-AC-W increased due to the non-graphitic carbonaceous species generated duringthe chemical activation. Consequently, the carbonaceous species were removed by the acid treatment,resulting in the disappearance of the peaks (Figure 3d) [27]. C-O and COOH peaks in XPS curves wereobserved to decrease and increase, respectively, after KOH chemical activation of ACs [28]. In thisstudy, the peaks related to the C-O and O-C=O also decreased and increased, respectively, after thechemical activation (Figure 3c,d; Table 2).

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Figure 3. X-ray photoelectron spectroscopy analysis of (a) S-AC, (b) H-AC, (c) C-AC-W, and (d) C-AC-A.

Table 2. Relative concentration of different groups in C 1 s peak based on X-ray photoelectron spectroscopy.

Surface Concentration (from C 1 s Peak) of (% at.):

B.E. (eV) (Assign) 283.6–284.2 284.6–284.8 285.5–285.6 286.7–286.8 288.8–289.0 290.6–291.1

(C-H) (C-C) (C-O/C-O-C) (C=O) (O-C=O) (π-π) S-AC 20.14 42.70 14.46 10.39 7.41 4.89 H-AC 21.13 39.57 18.69 8.85 6.55 5.21

C-AC-W 58.51 10.69 7.86 9.32 8.42 5.21 C-AC-A 16.01 39.25 14.20 10.60 9.87 10.08

After thermal treatment, the oxygen content of S-AC sharply decreased. The oxygen containing group was eliminated during the heat treatment. However, the ACs subjected to the chemical activation increased oxygen content due to the carbon oxidization. The total of analysis resulted in the chemically activated ACs were similar with S-AC and H-AC, but the C-AC-A differed (Table 3). The elements without nitrogen, carbon, hydrogen, sulfur, and oxygen could be assumed to be ash. Approximately 14% of total of C-AC-A increased compared to C-AC-W, meaning that ash was reduced by the acid treatment.

Figure 3. X-ray photoelectron spectroscopy analysis of (a) S-AC, (b) H-AC, (c) C-AC-W, and (d) C-AC-A.

Table 2. Relative concentration of different groups in C 1 s peak based on X-ray photoelectronspectroscopy.

Surface Concentration (from C 1 s Peak) of (% at.):

B.E. (eV) (Assign) 283.6–284.2 284.6–284.8 285.5–285.6 286.7–286.8 288.8–289.0 290.6–291.1

(C-H) (C-C) (C-O/C-O-C) (C=O) (O-C=O) (π-π)

S-AC 20.14 42.70 14.46 10.39 7.41 4.89H-AC 21.13 39.57 18.69 8.85 6.55 5.21

C-AC-W 58.51 10.69 7.86 9.32 8.42 5.21C-AC-A 16.01 39.25 14.20 10.60 9.87 10.08

After thermal treatment, the oxygen content of S-AC sharply decreased. The oxygen containinggroup was eliminated during the heat treatment. However, the ACs subjected to the chemical activationincreased oxygen content due to the carbon oxidization. The total of analysis resulted in the chemicallyactivated ACs were similar with S-AC and H-AC, but the C-AC-A differed (Table 3). The elementswithout nitrogen, carbon, hydrogen, sulfur, and oxygen could be assumed to be ash. Approximately14% of total of C-AC-A increased compared to C-AC-W, meaning that ash was reduced by theacid treatment.

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Table 3. Elemental analysis (EA) of prepared ACs.

Elemental Analysis (wt. %)

Nitrogen Carbon Hydrogen Sulfur Oxygen Total

S-AC N.D. 69.98 0.38 N.D. 2.27 72.64H-AC N.D. 73.43 0.33 N.D. 0.84 74.60

C-AC-W N.D. 71.30 0.34 N.D. 3.47 75.12C-AC-A N.D. 86.26 0.11 N.D. 2.30 88.68

Analysis condition: dry basis. N.D.: not detected.

The ash removal was also confirmed by the proximate analysis as shown in Figure 4. The ashcontent of S-ACs was 30%, which was slightly reduced through the thermal treatment and chemicalactivation. In particular, the ash content of C-AC-A was sharply reduced. The contents of C-AC-A wassimilar to the commercial ACs (especially coal-based ACs) [29]. Therefore, this suggests that the mostphysical properties of C-AC-A were recovered as fresh ACs.

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Table 3. Elemental analysis (EA) of prepared ACs.

Elemental Analysis (wt. %)

Nitrogen Carbon Hydrogen Sulfur Oxygen Total S-AC N.D. 69.98 0.38 N.D. 2.27 72.64 H-AC N.D. 73.43 0.33 N.D. 0.84 74.60

C-AC-W N.D. 71.30 0.34 N.D. 3.47 75.12 C-AC-A N.D. 86.26 0.11 N.D. 2.30 88.68

Analysis condition: dry basis. N.D.: not detected.

The ash removal was also confirmed by the proximate analysis as shown in Figure 4. The ash content of S-ACs was 30%, which was slightly reduced through the thermal treatment and chemical activation. In particular, the ash content of C-AC-A was sharply reduced. The contents of C-AC-A was similar to the commercial ACs (especially coal-based ACs) [29]. Therefore, this suggests that the most physical properties of C-AC-A were recovered as fresh ACs.

Figure 4. Proximate analysis of the ACs of S-AC, H-AC, C-AC-W, and C-AC-A.

The ashes usually were generated from metal ions in the waste water. The S-ACs were collected in the final process of WWTP. During this process, the various metal ions were adsorbed on the AC surfaces, so the water was purified. The adsorbed metal ions were not evaporated during both thermal treatment and chemical activation, and remained on the carbon surface and pores. The metal ions were converted into the ashes, which blocked the pores and covered the active carbon surface [17]. They reduced the surface area and also prohibited the utilization of ACs. The adsorbed metal ions were analyzed by XRF (Figure 5; Table 4). The Ca, Si, and Fe ions were mainly occupied on the S-ACs. In the H-ACs and C-AC-W, the metal composition was not significantly different. However, the composition and amount of metals changed in C-AC-A. In particular, Ca and Si were reduced compared to the others groups. Acetic acid (CH3COOH) could dissolve metal ions, including Ca and Si, changing the metal composition and reducing ash content as shown in Figure 5.

Figure 4. Proximate analysis of the ACs of S-AC, H-AC, C-AC-W, and C-AC-A.

The ashes usually were generated from metal ions in the waste water. The S-ACs were collectedin the final process of WWTP. During this process, the various metal ions were adsorbed on the ACsurfaces, so the water was purified. The adsorbed metal ions were not evaporated during both thermaltreatment and chemical activation, and remained on the carbon surface and pores. The metal ionswere converted into the ashes, which blocked the pores and covered the active carbon surface [17].They reduced the surface area and also prohibited the utilization of ACs. The adsorbed metal ions wereanalyzed by XRF (Figure 5; Table 4). The Ca, Si, and Fe ions were mainly occupied on the S-ACs. In theH-ACs and C-AC-W, the metal composition was not significantly different. However, the compositionand amount of metals changed in C-AC-A. In particular, Ca and Si were reduced compared to theothers groups. Acetic acid (CH3COOH) could dissolve metal ions, including Ca and Si, changing themetal composition and reducing ash content as shown in Figure 5.

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Figure 5. X-ray fluorescence (XRF) analysis of the chemical composition of ACs.

Table 4. Summarized XRF analysis of the ash of ACs.

XRF Analysis Results (wt. %)

Ca Si Fe S K Al Mn Ti P Cu Cr S-AC 31.2 23.5 18.2 8.0 5.8 5.02 2.6 2.3 1.9 0.9 0.6 H-AC 29.1 29.2 12.6 9.4 6.9 6.27 2.1 2.1 2.3 0 0

C-AC-W 34.3 31.1 13.0 0.6 7.9 8.77 2.0 2.4 0 0 0 C-AC-A 3.7 21.2 44.2 4.8 16.0 1.6 0 6.0 2.6 0 0

The regeneration of S-ACs using the KOH chemical activation increased the specific surface area, which was comparable to the fresh ACs. However, the metal ions present during the water purification process were adsorbed on the ACs and then converted to ashes. The generated ashes were not eliminated through the thermal treatment and chemical activation, leading to the negative effects. The ash removal was essentially required, and the acid washing shown in this study would be suggested.

Table 5. Toluene adsorption capacity of prepared ACs.

S-AC H-AC C-AC-W C-AC-A Commercial AC Toluene Capacity (g-toluene/g-AC) 0.033 0.100 0.091 0.154 0.142

The toluene adsorption capacities on the ACs were measured using the experimental setup as shown in Figure 1. The surface area, pore size, and functional groups on the surfaces of ACs are considered as key factors to affect adsorption mechanisms, leading to the various toluene adsorption performance [30–33]. First, the toluene adsorption capacities were largest at a higher surface area: C-AC-A > C-AC-W ≈ H-AC > S-AC, and the calculated adsorption capacity was 0.154, 0.091, 0.100, and 0.033 g-toluene/g-AC, respectively (Table 5). The toluene adsorption capacity calculated the integrated area divided by sample weight. In the previous reports [30,31], pore size distribution is also a key parameter to control the toluene adsorption. The toluene is usually adsorbed on the mesopore of ACs, and then diffused into the micropores. The mixture structure of micro- and meso-pores enhances their toluene adsorption capacity. If the pores are composed of only microstructures or too narrow, the adsorption capacity is decreased. The pores of ACs by chemical activation (C-AC-A) were composed of micropores (pore diameter

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than commercial ACs, leading to the replacement of fresh ACs. It gives the environmental andeconomic benefits.

Table 5. Toluene adsorption capacity of prepared ACs.

S-AC H-AC C-AC-W C-AC-A Commercial AC

Toluene Capacity (g-toluene/g-AC) 0.033 0.100 0.091 0.154 0.142

4. Conclusions

The spent activated carbons (S-ACs) from WWTP were regenerated by heat treatment and KOHchemical activation. The specific surface area was increased by thermal regeneration (H-AC), but itwas highly increased using the chemical activation (C-AC). The surface area of activated ACs wascomparable to the fresh ACs, but ashes generated during this process caused issued for regeneratedACs. The ashes initially accumulated during the water purification process. The metal ions wereadsorbed on the ACs, and then converted into ashes. The accumulated ashes blocked the pores andcovered the surface, leading to the negative effects. The chemically activated ACs were washed byacetic acid (C-AC-A) and compared to the ACs without acid washing (C-AC-W). The ACs with acidwashing increases 17% in available surface area, and the pore volume also increased. The C-AC-Awas applied for toluene adsorption. The toluene adsorption capacity of C-AC-A was even higher thanthe commercial ACs. The quality of regenerated ACs was improved comparing with the commercialACs, so it could replace the fresh ACs. Hence, the regenerated ACs preserve the environment andprovide benefits to the industries.

Author Contributions: Conceptualization, validation, investigation, writing—original draft preparation, reviewand editing, J.E.P. and S.Y.H.; methodology, and software, and data curation, G.B.L.; formal analysis, S.Y.H.;resources, supervision, project administration, and funding acquisition, B.U.H.

Funding: This subject is supported by Korean Ministry of Environment (Project No. 201902014) and the KoreaAgency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructureand Transport (Grant 19IFIP-B113506-04).

Conflicts of Interest: The authors declare no conflict of interest.

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http://dx.doi.org/10.1038/s41598-019-47100-zhttp://dx.doi.org/10.1007/s13762-019-02376-6http://dx.doi.org/10.1016/S0008-6223(00)00161-5http://creativecommons.org/http://creativecommons.org/licenses/by/4.0/.

Introduction Materials and Methods Preparation of Regenerated ACs Characterization of Prepared ACs Test for Adsorption of Toluene on the ACs

Results and Discussion Conclusions References